I tried the search for this and came up with some hits, but none that specific to my problem. My mods are in my sig. The only difference is I have Jesel SS rockers. The short history is this: About 2 months ago I took the car out for the last time this winter and was driving it around. When I came to a clover leaf I had the car in third at ~1600rpms and gave it some pedal, instead of picking up speed I got a series of backfires. Almost like a machine gun. I let off tha gas and drove it lightly home and no more backfires. The next time I started it and drove it the car would randomly backfire sitting at a light. Sometimes softly, a little 'putt', sometimes like a shotgun. And when I got on it, it would misfire, but I could drive through it. There was a section of lower rpm's that it would run rough in but if I went thorugh it with accelerator and got above 2k rpm's it was fine and ran smoothly all the way to redline. I pulled my spark plug wires, Taylor Spiro Pro's and measured them and only 1 wire measured good, ~350ohms. A few were 1 to 4 k ohms, and the rest were above 2 megs, the limit of my meter. I replaced them with a set of MSD SuperConductors and the misfires at part throttle was fixed.
However, I still get a random pop when idleing also. I am running NGK TR6iX's at 11.3:1 compression. I have also checked and made absolutely sure there are no leaks behind the MAF. I also cleaned the MAF thoroughly. I found the LSX manifold was sorta loose and I tightens it down nice and tight. I had the Jesel's on the car for around 300 miles before this happened. I just checked the pushrods, preload, and wipe and all are fine. THe wipe is right in the middle, although a little fat, but not bad. Also the idle is rough and hunts more than I remember it. I checked my springs and they all look fine. If I had a bent valve or bad lifter I would think I would get the popping all the time, not only after the car is warmed up >150 degrees F. Also, as soon as you come off idle the car is smooth again and no pops or bangs.
I'm thinking I might have a bad coil or 2 or more from driving around with bad wires for so long. It also could be my manifold (ported LSX 90) that has a bad seal internally, I've read they are a little flaky. Could be an injector(flow-matched 30lb/hr SVO's), but I would think it would pop above idle too.
I'm going to do a leakdown test this weekend if I can get the stuff, but does anyone know a good way to check the coil packs? I have EFILive 7.3, but the misfire info at idle is not reliable, at least in this case. There are too many misfire's that the pop is lost in the mix. I suppose I could let it idle until it pops and see if it is local to one side of the engine and if so swap the coil packs and wait again.
Again, the car pulls like a freight train and will rip the tires up at the top of third at WOT so I don't think it's a bent valve or bad spring but any ideas at this point would be welcome.
A couple of other things is when I log the LTFT's at idle they go to arounfd+12->+13 on one side, and when I get off idle the move around quite a bit more than they used to. When I had the car set up this summer, my LTFT's were only +- .5% and tracked together. I also checked my O2 sensors for burned leads, they were fine, and logging them the fronts bounce around happily, and the rears are up around .630V and steady.
Maybe somebody can give me a good direction to look at this and some suggestions as to a good way to track this down.
Sorry for the long post, I just wanted the history to be understood.

 

the tr6s are known for misfiring some but you might want to pull and check them,if the plug wires were bad that could have fouled a couple.

 

Sounds to me like you have some sorta sealing issue with the exhaust valves. Is it backfiring through the intake or the exhaust? You said it doesn't start until the motor is over 150 right? If that is the case, aluminum expands alot. Double check what you are supposed to set the lash to and go from there. Hopefully it is the simple.

 

Backfiring out the exhaust, only when the engine temp is over 150Deg F. And it is random. maybe one every 90 to 120 seconds. The car starts right up, and when you come off idle it won't backfire. I've run the car at 1200-1600rpm's for 5 mins. and no backfire. Then when you let it drop to idle, it will start to randomly pop and bang. Could a lean misfire cause a backfire?

 

Crank sensor.
Mine did the same thing. Randomly KABOOM at idle.

 

Did you replace it, or did you do a CASE relearn?

 

Ok, more info. I removed and made sure with RTV (sensor-safe) that everything from the air filter back to the throttle body is not leaking. Today I'm removing the FAST manfold and doing SpinMonster's fix. This was taken apart and ported then reassembled, so I am thinking a leak in the runners could do this. It can't hurt. If it's not the manifold or seals, then I am going to look at my coils, then plugs, then injectors. If none of that it looks like a leakdown will be next...

I checked for codes relating to the CKP and didn't have any, so I'm not thinking that right now...